Today's Date: 2019-07-22
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Scab Epidemic Risk Model 

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Plant Pathologist Commentary: Small Grains Disease Update 07/18/2019

Since the Fourth of July holiday temperatures and dew points have been unpleasantly high for small grains. Needless to say, the risk models for FHB have been high for all of Minnesota as the Northern Red River Valley has also received some much needed rain since my last report two weeks ago.

I have been busy with field days and plot tours across the northern half of the state and scoring diseases in the rye and winter wheat trials in the southern locations. It wasn't difficult to find FHB in the southern trials and even in some of the northern trials the first few FHB infections could be found.

The first symptoms of FHB take about 7 to 10 days to develop. This week, therefor, is a good time to start accessing how much damage FHB may inflict this year as a lot the spring wheat across the state headed between the 4th and 10th of July.

These early FHB infections have the most impact on grain yield. First the affected florets and spikelet will produce the tell-tale, chalky-white tombstone kernels. Secondly these early infections have the best chance to grow into the rachis is cut off the nutrient flow to the developing kernels in the florest and spikelets above the initial infected floret, thereby halting grainfill.

One key difference between varieties that are rated very susceptible to FHB and those that are rated moderately resistant, is their ability to slow the disease development into to rachis and cutting off the development of the grain above the initial infection.

And although FHB is really a monocyclic disease, i.e. the current infections will not produce spore that will infect the current crop, the crop remains susceptible until you have the crop in the bin. The spores will come from the same sources of inoculum that provided the ascospores for these first infections. The yield losses of these later infections, however, are much less dramatic and the concern is more the presence of DON in the grain.

Finally, these high temperatures and dew points are already taking their toll on the yield potential of this crop. Especially the high nighttime temperatures shorten the grain fill period, resulting in fewer kernels and lower kernel weight.

Important Note Important Note: Please review Terms of Use before utilizing this site. This Fusarium Head Blight (FHB, or 'scab') model is a pre-flowering model. It is meant to be applied using weather conditions during the week leading up to flowering. Therefore, the epidemic risk for a particular field should be determined by selecting a time coinciding with or just prior to the initiation of flowering in that field. A growth stage estimator is available on this website, but you should still monitor growth in your own fields to better estimate the date on which flowering begins.